Electric discharge device having spaced electrodes sealed to opposite end of envelope



June 25. 1968 C J. P. SMITH 3,390,296

ELECTRIC DISCHARGE DEVICE HAVING SPACED ELECTRODES SEALED TO OPPOSITE END OF ENVELOPE Filed March 14, 1966 3o 26 I F -l 2O I 24 64 I 5e so J Perry Smith INVENTOR.

AGENT.

United States Patent ELECTRIC DISCHARGE DEVICE HAVING SPACED ELECTRODES SEALED TO OPPOSITE END OF ENVELOPE J Perry Smith, Hawthorne, Califi, assignor to TRW Inc.,

Redondo Beach, Calif., a corporation of Ohio Filed Mar. 14, 1966, Ser. No. 534,071 11 Claims. (Cl. 313-182) ABSTRACT OF THE DISCLOSURE A lead-in structure for an electric discharge device includes a hollow, cylindrical envelope portion and an electrically conductive header sealed to the end of the'envelope portion by an epoxy resin. The header includes a stepped cylinder having a sealing flange abutting the end of the envelope portion and a recessed surface contiguous with the sealing flange. The epoxy resin is bonded hermetically to the end of the envelope portion, to the sealing flange, and to the recessed surface. An electrode joined to the header extends within the envelope portion.

This invention relates to electric discharge devices, such as electron tubes, gas-filled tubes and lamps and the like, and more particularly to structural improvements which facilitate hermetically sealing an electrically conductive lead-in member to an envelope portion and conductively joining the lead-in member to an internal electrode.

In electric discharge devices that are provided with internal electrodes, it is usually necessary to employ one or more electrically conductive lead-ins hermetically sealed to the envelope. The lead-in has an internally extending portion which is conductively joined to an internal electrode and an externally extending portion which serves as a terminal for connecting the electrode to a power supply.

It is generally advantageous to effect both the hermetic seal of the lead-in member and the conductive connection thereof to the electrode with the least possible application of heat. Heating should be minimized in order to avoid contamination, deformation, and dimensional changes of the parts, as well as to minimize the setting up of undesireable physical stresses which may weaken the structure or reduce the performance of the device.

Accordingly, an object of this invention is to provide a simplified lead-in structure for an electric discharge device or the like.

A further object is to provide a lead-in structure for an electric discharge device that can be assembled within close tolerances with a minimum application of heat.

The foregoing and other objects are realized in accordance with the invention through the provision of a lead-in structure in which a conductive lead-in member is joined to an electrode by a press fit. The electrode is mounted within an envelope portion by hermetically sealing the lead-in to the envelope portion by an epoxy resin, with the sealing operation being effected at a relatively low temperature just sufiicient to set the epoxy resin. Inasmuch as the only heating step is that required to effect the hermetic seal, and this is accomplished at relatively low temperature, the present structure avoids many of the shortcomings of the prior art.

3,390,296 Patented June 25, 1968 "ice The single figure is a longitudinal sectional view of a gas discharge lamp employing a lead-in structure according to the invention.

Referring to the drawing, there is shown a gas discharge lamp 10, including a cylindrical envelope portion 12 of glass or quartz or other light transmitting, electrically insulating material. The opposite ends of the envelope portion 12 are closed by two electrically conductive lead-in members or headers 14 and 16, each having the appearance of a button.

The first lead-in member 14 has a cylindrical base portion 18 of slightly larger diameter than the outside diameter of the envelope portion 12, an intermediate cylindrical portion 20 separated from the base portion 18 by a recessed portion 22, and a neck portion 24 projecting from the intermediate portion 20. The intermediate portion 20 is of slightly smaller diameterthan the inside diameter of the envelope portion 12. The neck portion 24 is of substantially smaller diameter than the intermediate portion 20 and is somewhat longer than both the base portion 18 and the intermediate portion 20.

In accordance with one aspect of the invention, the leadin member 14 is hermetically sealed to the adjacent end of the envelope portion 12 by means of an epoxy resin sealant 26. The top surface of the base portion 18 forms a sealing flange 28 to which the adjacent lip or annular end of the envelope portion 12 is joined by the sealant 26. The rim 30 of the intermediate portion 20 and the recessed portion 22 form sealing surfaces to which the inner wall of the envelope portion 12 is joined by the sealant 26. The sealant 26 preferably extends along a portion of the outer surface of the envelope portion 12 and the rim of the base portion 18 to provide additional sealing surface;

The second lead-in member 16 is similar in construction to the first lead-in member 14, and includes a base portion 32, intermediate portion 34, recessed portion 36 and neck portion 38. The lead-in member 16 is sealed to the adjacent end of the envelope portion 12 by means of an epoxy resin sealant 40 similar to sealant 26.

The second lead-in member 16 is provided with a first internal bore 42 which extends through the base portion 32 and terminates approximately midway into the intermediate portion 34. An exhaust tubulation 44, shown tipped off or closed at its outer end, is hermetically sealed within the bore 42 by an epoxy resin sealant 46. A smaller bore 48 extends the remaining distance through the intermediate portion 34 to provide communication between the interiors of the envelope portion 12 and the exhaust tubulation 44.

The two neck portions 24 and 38 are provided with longitudinal bores 50 and 52. In accordance with another aspect of the invention, a pair of spaced apart electrodes 54 and 56 are mounted within the neck portions 24 and 28 respectively. The electrode 54 may serve as an anode and the electrode 56 may serve as a cathode. The electrodes 54 and 56 include cylindrical pins 58 and 60, each forming a press fit within the longitudinal bores 50 and 52 respectively. The pins 58 and are slightly larger in diameter than the bores 50 and 52 and are of harder material than the neck portions 24 and 28 so that they can be forced into the bores 50 and 52 to form a tight press fit.

The electrodes 54 and 56 terminate in button-like electrode ends 62 and 64, into which the pins 58 and 60 are shown as pressed fit. The button-like electrode ends 62 and 64 and the ends of the pins 58 and 60 are preferably ground and polished smooth to form opposing, continuous, curved surfaces suitably contoured to smooth the electric field gradient therebetween.

The gas discharge lamp includes a gaseous filling, such as hydrogen, nitrogen, argon or the like, of desired pressure.

In assembling the gas discharge lamp 10, the electrodes 54 and 56 are pressed into the neck portions 24 and 38 of the lead-in members 14 and 16. The electrodes are preferably made of a material from which little or no vaporization will ensue during gaseous discharge therebetween. Thoriated tungsten or pure tungsten are preferred materials, although other materials may be used. While the electrodes 54 and 56 are shown as made in two parts which are press fit together, it is permissible to form them from one piece of metal. If made in two parts as shown, the button-like ends 62 and 64 may be made of the same material as the pins 58 and 60. Alternatively, the ends 62 and 64 may be made of one material, such as thoriated tungsten and the pins 58 and 60 of different material, such as pure tungsten.

Suitable epoxy resin is then applied to the sealing surfaces of the lead-in members 14 and 16 and exhaust tubulation 44, which at this stage is somewhat longer than as shown and is open at both ends. The epoxy resin may be applied by a brush, syringe, or squeeze tube, for example. The recessed portions 22 and 36 each form a convenient channel for holding a ring of the epoxy resin.

One example of a suitable epoxy resin is Resiweld No. 4, Type B, manufactured by H. B. Fuller Company of Los Angeles, Calif. This epoxy is a thermal setting epoxy, and is preferably mixed in the proportion of 50 parts resin and 1 part catalyst, by volume. This epoxy polymerizes at about 120 F.

The lead-in members 14 and 16 are made of a metal which is a relatively good electrical conductor, such as copper. Since high electrical conductivity metals have higher thermal coefiicients of expansion than the glasses or quartz which are used for the enveolpe portion 12, it is advantageous that the epoxy resin used for the sealants 26, 40 and 46 have a thermal coefiicient of expansion that is intermediate those of the metal of the lead-in members 14 and 16 and the material of the envelope portion 12. In this way, thermal stresses at the seals are minimized during the sealing process and during operation of the finished lamp.

In a preferred embodiment, the envelope portion 12 is made of a hard glass, such as lead borosilicate glass having a thermal coefiicient of expansion from 32 F. to 212 F. of 4 10- inch/inch/degree C. The lead-in members 14 and 16 are made of copper having a thermal coefiicient of expansion from 32 F. to 212 F. of 9 10 inch/ inch/degree C. The epoxy resin has a thermal coeflicient of expansion of 6 10- inch/inch/degree C. from 32 F. to 212 F.

While the exhaust tubulation 44 may be made of glass, like the envelope portion 12, where it is desired to have a high gas pressure within the lamp 10 that is higher than atmospheric pressure, the exhaust tubulation 44 may be made of copper rather than glass in order to facilitate sealoff. Seal-off of glass tubulation is diflicult, if not impossible, Where the pressure within the envelope is higher than atmospheric. However, tubulations of relatively soft materials such as copper are relatively easy to pinch-off by a pressing operation.

The epoxy resin coated lead-in members 14 and 16, with their electrodes mounted therein, are then assembled at both ends of the enveolpe portion 12, and the epoxy resin coated exhaust tubulation 44 is inserted within the bore 42 of the second lead-in member 16. The parts are conveniently assembled with the aid of a jig, not shown. The assembly is then placed in a baking oven to cure the epoxy resins and solidify the sealants 26, 40 and 46. A baking temperature of 120 F. and a heating time of sixty (60) minutes has been found satisfactory to produce good hermetic seals.

After the epoxy resin seals have been made, the discharge lamp 10 is exhausted of air through the exhaust tubulation 44, which at this time is open at both ends. When a suitable high vacuum has been attained, such as 5X10 millimeters of mercury, the assembly is baked out at 200 C. to degas the parts and the lamp is filled with a measured quantity of ionizable gas, such as hydrogen, nitrogen, argon or the like. For example, 2.1 cm. of hydrogen may be introduced, after which the exhaust tubulation 44 is closed oif at its lower end, as shown, to complete the manufacture.

It has been found that the resiliency of the epoxy sealants will absorb or cushion differences in the thermal coetficient of expansion between the glass and metal, which may occur during lamp operation.

In an operative embodiment, the following dimensions were used:

Length of envelope portion 12 ins" 2 Inside diameter of envelope portion 12 mm-.. 10 Outside diameter of enveolpe portion 12 do 13 Diameter of headers 14 and 16 in /2 Diameter of bores 50 and 52 do .059 Diameter of pins 58 and 60 do .060 Diameter of electrode ends 62 and 64 do .20 Spacing between electrode ends 62 and 64 cm 1 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A lead-in structure for an electric discharge device, comprising:

a hollow, cylindrical envelope portion;

an electrically conductive lead-in member including a stepped cylinder having a sealing flange abutting the end of said envelope portion and a. recessed surface contiguous with said sealing flange;

an epoxy resin bonded and hermetically sealed to the end of said envelope portion, to said sealing flange, and to said recessed surface; and an electrode disposed within said envelope portion and rigidly joined to said lead-in member.

2. The invention according to claim 1, wherein the material of said lead-in member has a higher thermal coefficient of expansion than said epoxy resin;

and said epoxy resin has a higher thermal coeiiicient of expansion than the material of said envelope portion.

3. The invention according to claim 1, wherein said lead-in member includes a neck portion free of epoxy resin projecting within said envelope portion along a longitudinal axis thereof;

and said electrode includes a supporting pin within said neck portion and aligned with said axis.

4. The invention according to claim 1, wherein said leading member is made of copper and said envelope portion is made of glass.

5. The invention according to claim 1, wherein said lead-in member is made of copper and said electrode is made of tungsten.

6. A gaseous discharge device, comprising:

a tubular portion of electrically insulating material;

a pair of metallic headers, each hermetically sealed to opposing ends of said tubular portion by an epoxy resin to form a sealed envelope;

each of said metallic headers including a stepped cylinder having a sealing flange abutting a respective end of said tubular portion, and a recessed surface contiguous with said sealing flange;

said epoxy resin being bonded to the ends of said tubular portion, to said sealing flanges, and to said recessed surfaces;

a pair of electrodes disposed within said envelope with opposing end surfaces in spaced apart relation;

each of said electrodes being joined to a respective one 10. The invention according to claim 6, wherein said of said headers; electrodes are aligned along a longitudinal axis of said and a filling of ionizible gas within said envelope. tubular portion.

7. The invention according to claim 6, wherein the ma- 11. The invention according to claim 6, wherein the 0pterial of said headers has a higher thermal coefficient of 5 posing end surfaces of said electrodes have convex curvaexpansion than said epoxy resin; tnre.

and said epoxy resin has a higher thermal coefiicient References Cited of expansion than the material of said tubular portion. UNITED STATES PATENTS 8. The invention according to claim 6, wherein said 2.141655 12/1938 KO 313 326X tubular Portion is made 0f glass; 10 2:491:631 12/1949 Wall e121? w 313326 X and Said made are made COPPeY' 2,879,323 3/1959 Nichols et al. 174 1s2 9. T116 invention according 0 claim 6, wherein said 2 923 79 2 19 0 Gerth 313 24 headers are made of copper;

and said electrodes are made of tungsten. DAVID J. GALVIN, Primary Examiner. 

